This invention relates to a sensor device of the kind specified in the pre-characterizing part of claim 1 and a method according to the pre-characterizing part of claim 19.
In known devices and methods, e.g. the optical absorption or the absorption power of a chemical substance, especially a fluid, (e.g. DE 197 17 145 C2, DE 100 63 678 A1) is determined as a physical value. A laser diode emitting a single mode is tuned through a predetermined wavelength range, in order to sweep through at least one characteristic absorption line in the spectrum of a fluid to be detected. The tuning of the laser diodes is effected by variation of its working temperature for example.
A problem with such sensor devices is represented by the small interaction length of the light with the fluid molecules to be detected. It is therefore already known also the increase the effective length of interaction of the sensor device by using a micro-resonator in the form of a photonic crystal, which simultaneously conflicts with the desire for miniaturisation. Either light emitted by an external light source, e.g. a laser, is passed through the photonic crystal through which the fluid flows (e.g. DE 100 63 151 A1) or the photonic crystal together with a laser active material are so combined in a laser that the gas flowing through the resonator directly affects the laser characteristics or the emission behaviour of the laser (e.g. DE 101 19 618 A1). The nature or the concentration of the fluid can be inferred from the change in the emission characteristic.
The detection sensitivity of such fluid sensors has however not been found sufficient for practical applications, especially when it is to be used for qualitative or quantitative detection of toxic fluids in particular, or fluids which are especially dangerous for other reasons.
In the case of a fluid sensor, the optical absorption as physical value effects attenuation of the intensity of the radiated light and thus an alteration of the emission behaviour of the laser.